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Football's age of adventure is over. Let us welcome the new astringency. Defending is back in fashion. Last season, lovers of the game yearned for a Champions League final between Arsenal and Real Madrid, a festival of flamboyance; this season, Europe's two best teams have been Chelsea and AC Milan, who between them have conceded just 33 goals in 67 league games.

Liverpool may disagree, but after the first legs of this season's Champions League semi-finals, these two remain on course to meet in the final. That's the thing about miserliness: it is so much more reliable.

Although the achievement of Bobby Robson's side in reaching the semi-final rather disguised the fact in England, football reached a nadir in the 1990 World Cup. It was negative, it was brutal, it was tedious. Fifa, the sport's world governing body, acted: the backpass was banned; the tackle from behind, which did so much to stifle and intimidate creative players, outlawed. Football slowly rediscovered the joy of attacking.

Italian sides never seemed wholly convinced and there were always the likes of Valencia flying the flag for caution, but the European Championships of 2000 was arguably the greatest international tournament of the modern era, full of open, skilful football.

Club competitions were dominated by the galácticos of Real Madrid and the cavaliers of Manchester United. When they met in the Champions League quarter-final in 2003, they produced an extraordinary spectacle, two games that will be talked about for generations, as Real won 6-5 on aggregate. And then they lost to Juventus in the semi-final, and a shadow fell across the sun.

The following summer, Greece won the 2004 European Championships, largely by dint of reinventing man-marking, against which everyone had forgotten how to play. And now Chelsea, too, are prospering with an approach that, if not defensive, at least ensures the defending is done well.

True, they have just had a 6-5 aggregate of their own in a Champions League quarter-final, but the difference between them and Real is that while the former Madrid coach Vicente del Bosque revelled in such scorelines, Chelsea manager José Mourinho abhors them. Stripped of both first-choice wingers on Wednesday against Liverpool, hatches were battened, midfielder Tiago recalled, and Mourinho was delighted to have a goalless scoreline to take into Tuesday's second leg.

That is not, as the Arsenal full-back Lauren apparently did, to dismiss Chelsea's imminent Premiership success as "boring", despite the truth in his manager Arsène Wenger's suggestion that Chelsea are not so pleasing on the eye as Arsenal. Wenger's side last season, though, were an aesthetic phenomenon. In possession they were majestic; out of it, well, the best that can be said is that they were not out of possession very often.

When they were forced to defend, and this was evident in the Champions League, they were found wanting. To go through a Premiership season unbeaten despite that flaw is testament to just how good they were with the ball.

About Chelsea, though, there is no such quibble. Mourinho predicted before Christmas that they would seal the Premiership title in Bolton, and should they win there this afternoon, they will do so. Everything he has done this season has had a similar authority.

There is nothing complex about his method. Mourinho is one of an increasing breed of managers who have not played the game to any great level. As a result, he has an analytical distance, unhindered by thoughts of what he did or of what happened to him. He has studied football almost as an academic discipline, and, although he denies modelling his side directly on any other, if his team is anything to go by, he appears to have come to believe that the acme of football are Fabio Capello's AC Milan of 1994. Perhaps no other side in the history of the game has married such a stern defensive base to such a wonderfully fluent attack.

Chelsea's back-four defends. Occasionally a full-back will advance over halfway, but it is rare. Terry Venables, the former England coach, observed after a game at Stamford Bridge this season that he had watched Paulo Ferreira - whose recent absence has exposed an unexpected vulnerability - throughout the first half: he crossed the halfway line twice and touched the ball just eight times. Even in a game Chelsea were dominating, he remained disciplined.

In front of that back-four sits Claude Makelele, a destroyer who rarely ventures forward, his solidity giving the rest of the midfield licence to roam, just as Marcel Desailly allowed the talents of Zvonimir Boban and Dejan Savicevic freedom in Capello's Milan. Frank Lampard, his remarkable stamina allowing him to get back and provide extra cover, can surge to join the attack, while two of Damien Duff, Joe Cole and Arjen Robben can sparkle on the flanks. Sometimes Tiago or Alexey Smertin is included, an extra engine to protect the defence; more recently Mourinho seems to have preferred Eidur Gudjohnsen.

The shape shifts from 4-3-2-1 (with the two playing wide) to 4-2-3-1 to 4-1-4-1, but the nuances are less important than the split nature of the midfield, which gives both defensive sturdiness and an attacking fluidity. At their best, Chelsea can hit sides from five different positions; even at their worst - and Wednesday was as bad as it has been this season - their defence is adamantine.

Where Mourinho does differ from Capello's Milan is in the use of a target man. Although he has scored 16 goals this season, Didier Drogba is not one of the world's natural finishers, nor is he a great technician. Yet he is quick, strong and brave, and if £24m seems a lot to pay for a turbo-charged Niall Quinn, he gives Chelsea an extra dimension. From a defensive point of view, if they need to clear their lines, he provides an outlet. Going forwards, he gives another option; it was remarkable in the game against Bayern how the Brazilian centre-back Lucio, so adept on the floor, crumbled when placed under aerial bombardment. Drogba is a pragmatic rather than a pretty player, but Mourinho is a pragmatic coach.

Amid his gifts as a speaker and a motivator, Mourinho's tactical acumen is often overlooked. Chelsea may not excite and enthral as other recent teams, but they are emphatically modern.

Mourinho, though, is no great innovator: rather, his method is a bricolage of techniques garnered from elsewhere. Nevertheless, his reinterpretation of the split midfield and the target man and his championing of a defence that merely defends, makes him the presiding spirit of football's conservative new age. Backwards, sometimes, is forwards.